Fluid dynamic bearing devices are for supporting a rotation-side component of a pair of components which relatively rotate by a film of a fluid occurring in the bearing gap. This type of bearing devices have the features such as high-speed rotation, high rotational accuracy and reduced noise, and are suitably used as bearing devices for motors mounted on various electrical machinery and apparatuses typically including information appliances, more specifically, as bearing devices for spindle motors in disk drives used in magnetic disk apparatuses such as HDD, optical disk apparatuses such as CD-ROM, CD-R/RW and DVD-ROM/RAM, magneto-optic disk apparatuses such as MD and MO, or as bearing devices for polygon scanner motors of laser beam printers (LBP), collar wheel motors of projectors, and bearing devices for motors such as fan motors.
For example, in fluid dynamic bearing devices integrated into spindle motors for HDD, one or both of radial bearing portions which support the shaft member in the radial direction, and a thrust bearing portion which supports the shaft member in the thrust direction are constituted of hydrodynamic bearings in some cases. As is often the case with such bearing devices, hydrodynamic grooves constituting a hydrodynamic pressure producing part are formed on either of the inner circumferential surface of a bearing sleeve and the outer circumferential surface of the shaft member opposing this, and a radial bearing portion is formed in a radial bearing gap between both surfaces (for example, refer to Japanese Unexamined Patent Publication No. 2003-239951 as Patent document 1). A hydrodynamic pressure producing part (thrust hydrodynamic pressure producing part) such as the hydrodynamic grooves provided on both end faces of a flange portion of the shaft member or end faces of components which oppose each other across the thrust bearing gap (for example, end faces of a bearing member, inner bottom faces of a housing, etc.) produces pressure in the thrust bearing gap by the hydrodynamic effect of a lubricating fluid, whereby the thrust bearing portion is formed (for example, refer to Japanese unexamined Patent Publication No. 2002-61641 as Patent document 2).
For these structural components of fluid dynamic bearing devices typically including the shaft member, high processing accuracy and assembly precision are required to ensure high rotational performance as information appliances become more sophisticated. Meanwhile, demand for reduction in cost for fluid dynamic bearing devices is ever increasing.
As a measure for reducing cost in manufacturing fluid dynamic bearing devices, forming the shaft member of a resin is suggested (for example, refer to Japanese Unexamined Patent Publication No. 2000-330066 as Patent document 3).
However, when the shaft member is formed of a resin, the shaft member expands radially outwardly as temperature rises, and the radial bearing gap may be consequently reduced. One may consider forming the bearing sleeve of the same kind of resin to maintain the bearing gap, but it is difficult to form parts made of a resin at high accuracy since the molding dimensions (dimensions of molds) of such parts need to be set considering shrinkage during molding. When the components which form the bearing gap are both formed of a resin, it is even more difficult to obtain the bearing gap at high accuracy because of the influence of molding dimensional variation of both components. Of course, the problem of this type can occur when components on the bearing side such as the bearing sleeve are formed of a resin in order to achieve cost reduction.
Moreover, cost reduction/sealing property tradeoffs entail the following problem: that is, since the various kinds of motors mentioned above extremely dislikes contamination by the lubricant, a sealing gap for preventing leakage of the lubricant is usually provided at the opening of the bearing member. Furthermore, in a region which is radially adjacent to the sealing gap, a lubricant reservoir for providing the bearing gap (radial bearing gap) with the lubricant is provided in some cases. By providing this lubricant reservoir, reduction of bearing life due to shortage of the lubricant is prevented. The above-mentioned sealing gap and lubricant reservoir are formed, for example, between a separate component fixed on the bearing member and the shaft member (for example, refer to Japanese Unexamined Patent Publication No. 2000-235160 as Patent document 4).
In the mean time, with recent trend of lower prices of information appliances and the like, demand for reduction in cost for fluid dynamic bearing devices is ever increasing. However, as in the above Patent document 4, when the sealing gap and the oil reservoir are formed between a component provided separately from the bearing member and the shaft member, the number of parts and assembling man-hours are increased, which inevitably increases the cost.
Moreover, accuracy/cost tradeoffs in the hydrodynamic pressure producing part entail the following problem: That is, as a fluid dynamic bearing device of this type, for example, Japanese Unexamined Patent Publication No. 2005-321089 (Patent document 5) discloses a fluid dynamic bearing device (hydrodynamic bearing device) which supports the shaft member in the radial direction in a non-contact manner by the hydrodynamic effect of the fluid occurring in the radial bearing gap between the inner circumferential surface of the bearing and the outer circumferential surface of the shaft member. Two (upper and lower) regions which serves as a radial bearing face are provided separately in the axial direction on the inner circumferential surface of the bearing, and the hydrodynamic grooves arranged, for example, in a herringbone pattern, are formed in the two regions.
Japanese Unexamined Patent Publication No. 2000-81028 (Patent document 6) discloses a hydrodynamic bearing device in which the bearing is formed of a resin in an attempt to improve its sliding property with the shaft member and moldability of the bearing. In this bearing, the hydrodynamic grooves are formed simultaneously with molding injection of the bearing, and therefore the hydrodynamic grooves can be easily formed.
Forming hydrodynamic grooves simultaneously with injection molding of the bearing can be achieved, for example, by forming a molding portion corresponding to the shape of hydrodynamic grooves on a forming mold, and transferring the shape of the molding portion onto the inner circumferential surface of the bearing in molding of the bearing. In this molding method, however, the resin gets into the recesses on the molding portion of the mold and cures. Therefore, when the mold is released from the inner periphery of the bearing, the molding portion of the mold and the hydrodynamic grooves of the bearing may interfere with each other, thereby damaging the hydrodynamic grooves.
Moreover, the dimensional stability/cost reduction tradeoffs of the fluid dynamic bearing device having a resin portion entail the following problem: That is, for example, Japanese Unexamined Patent Publication No. 2003-56552 (Patent document 7) discloses a bearing part used for a fluid dynamic bearing device of this type which is produced by insert-molding a cylindrical metal part (electroformed part) having an inner hole using a resin.
A bearing part made of resin has the advantage of being lighter and more economical than that made of metal, while it has the disadvantage of large dimensional change due to thermal shrinkage and other causes. In the bearing part of Patent document 7 mentioned above, the cylindrical metal part (electroformed part) is retained on its inner periphery in an attempt to suppress a change in the radial dimension of the bearing part made of resin and improve circularity and the dimensional accuracy of the inside diameter. However, a change in the axial direction of the bearing part is unavoidable, and thus sink marks and other problems due to a difference in a dimensional change occur, which may adversely affect the axial dimension and accuracy of the bearing face.    Patent document 1: Japanese Unexamined Patent Publication No. 2003-239951    Patent document 2: Japanese Unexamined Patent Publication No. 2002-61641    Patent document 3: Japanese Unexamined Patent Publication No. 2000-330066    Patent document 4: Japanese Unexamined Patent Publication No. 2000-235160    Patent document 5: Japanese Unexamined Patent Publication No. 2005-321089    Patent document 6: Japanese Unexamined Patent Publication No. 2000-81028    Patent document 7: Japanese Unexamined Patent Publication No. 2003-56552